1. Field of the Invention
The present invention generally relates to power saving circuits and specifically relates to a power saving circuit for solenoid drivers, especially those which are battery powered.
2. Description of the Prior Art
Solenoids are comprised in their simplest form of a coil and an armature which is free to move within the coil. The armature is normally spring loaded away from the energized position such that when a power pulse is applied to the coil, the armature is pulled into the energized position and in moving can do useful work. It is known that once the solenoid has moved to the end of its operating stroke, no further work will be done by the armature.
Because the amount of current flow through the coil determines the strength of the magnetic field acting upon the armature and the voltage applied to the coil determines the current flow through the coil, a battery which has been recently recharged or exchanged for a fresh battery will cause the greatest magnetic field acting on the armature. Generally however, toward the end of the useful life of a battery, the voltage is at a minimum, the coil""s magnetic field is at a minimum and the acceleration of the armature is less. As a result, the duration of voltage application to the coil must be sufficiently long in order to permit the armature, accelerating at a slower rate, to complete its operating stroke. Thus, with a fixed duration voltage pulse applied to the coil, a relatively longer duration is required. Unfortunately, when the battery is freshly charged or new, the time needed for completion of the operating stroke is substantially less than the fixed duration needed for the weakest battery in order to complete the desired solenoid motion. Thus, with a fresh battery, after the operating stroke has been completed, the coil. due to the longer duration pulse, continues to be energized, thereby wasting battery power.
One preferred application of battery powered solenoids is as a Braille impact printer. The solenoid""s operating stroke is utilized to drive a pin which impacts and embosses a paper target so as to produce Braille characters readable by touch. The solenoid""s function, therefore, is to produce enough impact energy on the target to suitably emboss the target such that the impact can be xe2x80x9creadxe2x80x9d by feel.
For portable Braille printers small enough for a student to carry and use in a classroom, the weight and size must be minimized. As a result, the battery size and its energy storage capacity is limited. Efficiencies in the solenoid driver circuit are magnified because the battery capacity is generally required to operate up to six solenoid impacts to form each Braille character. A Braille character embodies a matrix of embossments that are three vertical spots by two horizontal spots.
If the average Braille character requires three solenoid impacts and the average Braille word length is 4.2 letters long (this includes punctuation characters), and a desirable word quantity between battery charges is approximately 8000, it will be seen then that a total of 100,800 solenoid impacts will be required per battery charge. The total battery energy consumed by the solenoid is a multiplication of the battery voltage, the solenoid current, the on time of the electrical pulse and the number of pulses supplied to the solenoid.
In conventional Braille printers, the duration of the electrical pulse is set, for example, to be 10 milliseconds long in order to permit sufficient embossment energy when the battery is at its lowest usable voltage. However, with the fresh battery (and the resultant increased acceleration and reduced operating stroke) the embossment function can be performed in 6 milliseconds. As a result 4 milliseconds (or 40%) of the consumed energy is wasted (generally as heat) in coil xe2x80x9conxe2x80x9d time when the operating stroke has been completed where a fixed pulse duration of 10 milliseconds is employed by the electronic driver circuit.
It is therefore an object of the present invention to control energization pulses applied to a solenoid coil so as to reduce energy consumed.
It is another object of the present invention to provide a solenoid coil energizing pulse which terminates at about the end of the operating stroke of the solenoid.
It is a further object of the present invention to recover energy which is stored in the magnetic flux created by the solenoid coil and store this otherwise wasted energy.
The above and other objects are achieved in accordance with the present invention by sensing the current flow through the solenoid coil and when the current rises above the current demand during the operating stroke as when the armature ceases its movement, current flow to the coil is cut off. Current flow to the coil can be sensed by placing a small resistor in series with the coil and current flow through that coil can be controlled by one or more power transistors also in series with the coil. In one embodiment, a microprocessor is programmed to monitor the solenoid coil current flow such that when an inflection point is reached (indicating completion of the armature""s operating stroke), current flow through the coil can be terminated. In another embodiment, because after termination of the operating stroke, coil current will rise above the maximum during the operating stroke, when the sensed coil current increases above the operating stroke maximum current flow is terminated. In a further preferred embodiment, the utilization of power transistors to isolate the coil permits diodes to provide current induced in the coil during the collapsing magnetic field (after the power transistors have been turned off) to be conveyed back to the battery, thereby saving battery power which would otherwise be lost.